Buildings or rooms comprising commercial kitchens, research laboratories, and industrial installations often include one or more exhaust fans that vent smoke, steam, and other air-polluting substances from areas where cooking units, chemical process, or manufacturing operations are located. To replace the exhausted air, heated makeup air devices are used to heat and introduce outside air into the building or room. These heated makeup air devices basically consist of a duct structure open to both the outside atmosphere and the building or room, a fan for blowing air through the duct structure into the building or room, heating units to heat the outside air as needed, and a control system for activating and monitoring the heater makeup air device as needed.
In some situations, amount of heating needed to heat outside air before introducing the air into the building or room varies widely over relatively short periods of time. For example, in some climates and at some times of the year, outside air temperatures may be very low in the morning as compared to the desired air temperature within the building or room. However, later in the day, the outside air temperature may rise considerably, perhaps up to about the desired air temperature in the building or room. At night, outside temperatures may again drop significantly. This situation requires highly variable heating rates throughout the day, and it is often the case that heated air makeup units are sized based on the highest heating demand. Likewise, venting requirements may vary over relatively short periods of time. For example, in a restaurant the intensity of cooking activities varies as dining customers come and go. While during a meal rush, venting of cooking fumes is done at a high rate and heated makeup air is consequently needed at a high level, during another period there may be little need for venting and the demand for heated makeup air is consequently low.
In most cases the air in the building or room is conditioned by a building or room heating system that is not interconnected with the heated makeup air unit The degree to which a heated air makeup device functions adequately can have a major bearing on heating load and heating load changes on the building or room air heating system. If heated makeup air is provided at temperatures varying even a little from the control target temperature of the building or room heating system, inefficient cycling of the building or room heating system can occur. However, providing precise control of the temperature of heated make up air using a very high capacity heating unit can be difficult and expensive. Moreover, actively varying the heating rate of a large heated air makeup unit often results inefficient energy conversion in the unit.
There is a need for heated air makeup devices which provide precise control of makeup air temperature and can operate efficiently over wide heating rate ranges.